You should not include an exchange rate increase in your profit calculations. If you pay 0.04BTC (or the equivalent in another currency) for a device that will only produce 0.01BTC (after operating costs) in its lifetime, then it is not profitable. If you want to bet on bitcoins increasing in value over other currencies, then just buy bitcoins. This assumes your only motivation is profit.
I was going to write a bunch of clever calculations but you are right. It is actually more sensible to just exchange the $15 to 0.03 btc.
But if you really wanted to make a profit with a mining device of some kind, how would you be able to gauge the performance? Like "price per ghash per watt < x"? Did anyone make an online calculator for that?
-Michael